The present invention relates to a method for fabricating a capacitor including an insulating film, containing a titanium oxide, as a capacitive insulating film and a method for fabricating a semiconductor device including such a capacitor. More particularly, the present invention relates to respective methods for fabricating capacitor and semiconductor device using strontium titanate deposited at 400xc2x0 C. or less.
First, known methods for depositing an insulating film containing a titanium oxide will be described. In this specification, strontium titanate is exemplified as a typical material for an insulating film containing a titanium oxide. According to reported methods, strontium titanate is deposited by metalorganic chemical vapor deposition (MOCVD), sputtering, sol-gel process or the like.
The relative dielectric constant of a strontium titanate film (at 100 or more) is larger than that of an SiN film or an SiO2 film. Accordingly, if a strontium titanate film is used as a capacitive insulating film for a capacitor, then the capacitance per unit area can be increased. In view of this advantage, the application of a strontium titanate film to a capacitor for a DRAM or a microwave monolithic IC (MMIC) requiring a high degree of integration is now under consideration.
In the field of mobile communication, the frequency range applicable to MMIC""s is expected to expand from the vicinity of 1 GHz for cellular phones to reach an even higher frequency in the near future. Thus, a capacitor, having capacitance (e.g., 10 pF or more) large enough to ground a circuit at a radio frequency, needs to be integrated with an IC so as to occupy as small an area of the IC as possible. Also, in order to operate a field effect transistor at a radio frequency, a compound semiconductor substrate including an active layer formed by epitaxy (hereinafter, such a substrate will be referred to as an xe2x80x9cepitaxial substratexe2x80x9d) should be desirably used for the active region of the transistor. This is because if an FET is formed by ion implantation, the resulting thickness of the channel region thereof and the concentration of the dopants implanted thereto are limited and not suitable for high-speed operation.
Taking these factors into consideration, the integration of a large-capacitance capacitor on an epitaxial substrate has increased its importance for the implementation of an MMIC operative at a radio frequency. However, an epitaxial substrate is thermally unstable, and therefore poorly resistant to a heat treatment conducted at a temperature higher than the epitaxy temperature thereof. Also, in a substrate in which a plurality of layers have been formed by heteroepitaxy from dissimilar materials, the heat treatment should be performed at a temperature low enough to suppress the deterioration at the heterojunctions thereof owing to thermal counter diffusion. Accordingly, in order to suppress the deterioration of a thermally unstable epitaxial substrate, a strontium titanate film should be desirably deposited at a lower temperature for a shorter period of time.
However, if a strontium titanate film is deposited at a temperature lower than the epitaxy temperature thereof by the prior art techniques, the following various problems happen.
In accordance with an MOCVD technique, organic metals supplied do not thermally decompose at a lower temperature and therefore films of excellent quality cannot be obtained. Thus, if a strontium titanate film is to be deposited by a conventional MOCVD technique, then the temperature of the substrate should be held at about 650xc2x0 C. during the deposition.
According to a sol-gel process, a coated film is crystallized while the sintering temperature is ordinarily set at about 800xc2x0 C. If the sintering temperature is lowered, however, the molten sources of strontium and titanium are not sufficiently crystallized. Thus, it is difficult to obtain a high-quality film.
Because of these reasons, RF sputtering is currently used in most cases for the deposition of a strontium titanate film on an epitaxial substrate.
With an RF sputtering technique, the deposition rate of a strontium titanate film is proportional to RF power applied. Nevertheless, if the RF power density is excessive, then the strontium titanate target cannot be cooled sufficiently. As a result, the target is damaged. Since it is hard to apply excessively high RF power, the deposition rate of a strontium titanate film cannot be increased satisfactorily. In addition, if extraordinarily high RF power is applied, then the properties of the epitaxial substrate deteriorate owing to the damage resulting from sputtering. Accordingly, during the deposition of a strontium titanate film, RF power applied cannot exceed a certain value, which requires a long time to deposit the strontium titanate film. The deposition process takes such a long time in accordance with RF sputtering. Thus, the deposition temperature should be even lowered so as not to deteriorate the properties of the epitaxial substrate. Nevertheless, if the deposition temperature is lowered, then the relative dielectric constant of the strontium titanate film deposited adversely decreases. A film having a low relative dielectric constant is not qualified as a capacitive insulating film for a capacitor.
The same argument can be constructed whenever a substrate or an integrated circuit has a structure with thermally deteriorative properties, even if an epitaxial substrate is not used.
A prime object of the present invention is to provide a method for fabricating a capacitor, in which a titanium oxide insulating film showing a high relative dielectric constant can be formed on a structure with thermally deteriorative properties, like an epitaxial substrate, while suppressing such deterioration.
A method for fabricating a capacitor according to the present invention includes the steps of: forming a lower-level electrode layer over a structure having thermally deteriorative properties; depositing an insulating film, containing a titanium oxide, on the lower-level electrode layer at a deposition temperature of 400xc2x0 C. or less; conducting a heat treatment at a temperature higher than the deposition temperature and lower than 500xc2x0 C. after the insulating film has been deposited; and depositing an upper-level electrode layer on the insulating film after the heat treatment has been conducted.
Another method for fabricating a capacitor according to the present invention includes the steps of: forming a lower-level electrode layer over a structure having thermally deteriorative properties; depositing an insulating film, containing a titanium oxide, on the lower-level electrode layer at a deposition temperature of 400xc2x0 C. or less; depositing an upper-level electrode layer on the insulating film; and conducting a heat treatment at a temperature higher than the deposition temperature of the insulating film and lower than 500xc2x0 C.
Still another method for fabricating a capacitor according to the present invention includes the steps of: forming a lower-level electrode layer over a structure having thermally deteriorative properties; depositing an insulating film, containing a titanium oxide, on the lower-level electrode layer at a deposition temperature of 400xc2x0 C. or less; depositing an upper-level electrode layer on the insulating film; working the upper-level electrode layer; and conducting a heat treatment at a temperature higher than the deposition temperature of the insulating film and lower than 500xc2x0 C.
Yet another method for fabricating a capacitor according to the present invention includes the steps of: forming a lower-level electrode layer over a structure having thermally deteriorative properties; depositing an insulating film, containing a titanium oxide, on the lower-level electrode layer at a deposition temperature of 400xc2x0 C. or less; working the insulating film into a desired capacitor shape; conducting a heat treatment at a temperature higher than the deposition temperature of the insulating film and lower than 500xc2x0 C.; and forming an upper-level electrode layer on the insulating film.
In accordance with the present invention, a method for fabricating a semiconductor device, including a transistor and a capacitor on an epitaxial substrate, is also provided. The process of fabricating the capacitor includes the steps of: forming a lower-level electrode layer over an epitaxial substrate; depositing an insulating film, containing a titanium oxide, on the lower-level electrode layer at a deposition temperature of 400xc2x0 C. or less; and depositing an upper-level electrode layer on the insulating film. A heat treatment is conducted at a temperature higher than the deposition temperature and lower than 500xc2x0 C. after the insulating film has been deposited and before the upper-level electrode layer is deposited.